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The 17 essential elements
are:
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Potassium
(K)
Form
used by plants:
K+
Important
functions:
-
Regulates
water status and activation of many enzymes
-
Involved
in photosynthesis and respiration
-
Involved
in the formation and translocation of sugars, proteins,
starch, and plant growth hormones
-
Required
in stomata opening and closing
-
Promotes
root growth
-
Increases
plants resistance to disease and improves winter hardiness
-
Fruit
size (fruits are strong sinks for K), color, and acidity
are related positively to potassium concentrations
Ideal
foliage range for apple leaves:
-
1.3-1.75%
-
< 1.2%
may mean K is limiting
-
Young
trees and nonbearing trees have higher leaf K then mature
cropping trees. Levels strongly influenced by crop
load, tending to be higher with light crops and decreasing
as crop load increases.
-
Levels
of leaf K for optimal crop quality is also dependent
upon the nitrogen status of leaves
ex. McIntosh 1.0-1.25 parts N to 1.0 part K are adequate
Delicious 1.25-1.5 N/K are adequate
Therefore, a Delicious sample that has 2.4% N should contain between
1.6-2.0% K.
Crop
requirement:
125-175
lbs/Ac/year which is equivalent to 150-210 lbs K2O.
60-90
lbs K are removed from orchard each year by crop.
This
value will also depend upon N status and variety. For example,
for varieties such as Delicious, ratios of 1.25 to 1.50
(N:K) appears to be adequate. This
translates to the following, a Delicious sample containing
2.4% N should contain between 1.6-2.0% K.
Ideal
soil range:
150-250
ppm or 0.4-0.6 meq/100 g soil (values based on the ammonium
acetate extraction method. If sodium bicarbonate method
is used, values may be slightly lower).
low: < 150
ppm (< 0.4 meq/100 g soil)
high:
250-800 ppm (< 0.6-2.0 meq/100 g soil)
excessive: > 800
ppm (> 2.0 meq/100 g soil)
Orchard
soils low in K are rare. The relationship between soil tests, tissue tests and
response of fruit trees to K fertilizer has not been adequately
determined.
K
is strongly sorbed by soil components and thus is not readily
mobile in soils.
Best
indicators:
“Leaf
analysis in conjunction with soil testing of samples collected
from both the topsoil and subsoil provides the best estimate
of the amounts of potassium that are available for uptake
by fruit trees.” (From Tree Fruit Nutrition book)
Mobility
in plant:
Remobilized
within plant
Deficiency
symptoms:
-
Symptoms
are visible when the concentration is below 0.75% of
leaf dry weight. Leaves
containing 1% K do not show visible signs but fruit will
be smaller or poor color development occurs.
-
Older
leaves are affected first. Leaf scorch will show up first.
Undersides of leaf margins may become darkened or browned
(etching). Slow growth, tip and marginal chlorosis/necrosis
advancing toward the midrib may also occur. Some species
show a recurving of petioles along with upward rolling
of leaf blades. Necrotic speckling and scorching of leaves
is also common.
-
Shoots
are slender and spurs are weak.
-
Fruit
will be smaller, poorly colored and low in acidity (lacks
flavor). Fruit may hang on tree after leaf fall. Trees
may be more susceptible to winter injury and blossoms
to frost injury (also if high N/K ratio exists).
-
Time
of fruit maturation is delayed.
Increased
risk of K deficiencies on:
Excess
problems/Interactions with other elements:
-
Magnesium,
manganese, and calcium deficiencies become more pronounced
with excess K.
-
High
levels of Ca can limit K uptake in some soils.
-
An
adequate soil supply of boron must also be maintained
in order to optimize the utilization of K (to support
active root development for K uptake).
Updated
July 13, 2004
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